The present invention relates to scanning probe microscopes and more particularly to an apparatus and method for producing a measurement of the surface representative of a parameter of the surface other than topography or for performing a task on the surface. Scanning probe microscopes such as a scanning tunneling microscope or an atomic force microscope operate by scanning a probe over a surface in which the probe is very close to the surface, lightly contacts the surface, or taps on the surface.
In a scanning tunneling microscope, the tip is at a distance of just a few atoms from the surface in order for a tunneling current to flow between the probe tip and the surface. The tunneling current is either measured to represent the distance between the probe and the surface or more generally used in a feedback system, which regulates the vertical height of the probe, to keep the current and therefore the distance of the probe from the surface constant. The feedback signal therefore is a measurement of the topography of the surface. In an atomic force microscope the tip may be mounted on a bendable arm and therefore small deflections of the arm are measured in order to detect the profile of the surface under study. Alternately, a feedback system may be used to maintain the probe force constant on the surface and with the feedback signal representing the topography of the surface.
Both types of microscopes described above are variations of a general device referred to as a scanning probe microscope. Originally, scanning probe microscopes only used the two types of interactions described above, which are specifically the tunneling current or the contact force with the atomic force microscope. These types of interactions were used to adjust the height of the probe to trace the topography of the surface.
There have been a number of recent developments which include the use of other types of interactions between the probe and the surface so as to attempt to form different types of measurements or images of the surface. For example, it may be desirable to produce images of a surface representative of parameter such as Van der Waals forces, magnetic forces, electric forces, ionic conductance, electro-chemical activity and light intensity, wavelength or polarization. Since these new types of interactions measure parameters of the surface other than the topography of the surface, it is difficult to measure these new types of interactions while at the same time measuring topography.
The prior art scanning probe microscopes which have tracked the surface of a sample with a probe tip by sensing some parameter have included, as indicated above, tunneling current, contact force, Van der Waal attractive force, magnetic force, electro-static force, ionic conduction, electro-chemical activity and light intensity, wavelength or polarization. Some of these parameters such as tunneling current and contact force are generally easy to sense and are representative of the topography of the surface.
Others of these parameters, such as the magnetic force, are more difficult to detect or may not be directly related to the topography of the surface. This causes any measurement signals, such as feedback signals, responsive to these parameters, to either be marginal or unstable and not useful as a position signal. For example, some of these parameters are not continuous across the surface, i.e. the magnetic force may vary over a surface, disappearing or changing direction from down to up. Therefore, any position signal from these parameters is not stable and so the probe is not able to track the surface using these other parameters.
There are times when it is desirable to provide an image of the surface, representative of parameters other than topography and therefore, it would be advantageous when measuring these parameters, other than topography, which may be weak or discontinuous to not rely on these interactions for position information to control the height of the probe over the sample surface. It would therefore be advantageous for the measurement of such other parameters to move the probe a known distance away from the surface at all points along its contour while measuring these other parameters. For example, such a fixed separation is useful when measuring electro-chemical currents on a fluid covered surface. In this case, the desired spacing is too large to use tunneling currents to control the probe height and even if the spacing were reduced, the electro-chemical and tunneling currents would be combined so as to confuse the position control system.
One important example of a desirable parameter for measurement, other than topography, is the measurement of magnetic fields at a sample surface. One prior art attempt as suggested by Rugar and Wickramasinghe, Appl. Phys. Lett. 52, Jan. 18, 1988, p244, included vibrating a magnetic probe or tip above the surface and detecting the change in the frequency of vibration due to the sample. The sample caused both magnetic forces and Van der Waal attraction of the probe and so the feedback data contained both magnetic and probe height information. For many samples, these forces are extremely weak and give a poor feedback signal which causes the probe to hit and stick to the sample or drift away from the surface. As a result, the technique as suggested by Rugar and Wickramasinghe has not found widespread use.
Another technique for measuring the magnetic fields is that suggested by Moreland and Rice, which uses a tunneling microscope with a flexible magnetic probe or tip supported on a cantilever. The feedback signal is a tunneling current which is used to keep the tip just above the surface. Magnetic attraction pulls the flexible tip toward the surface and the position control system then lifts the tip back into position by bending the cantilever. Thus, the magnetic field patterns appear to be raised and lowered regions of the surface. Unfortunately, this mixing of position and magnetic data is a disadvantage to the Moreland and Rice system since inaccuracies are introduced. In addition, the sample must be electrically conducting to obtain a tunneling current and this is a disadvantage for many important magnetic media such as magnetic tape or magneto-optical disks which are not conducting.
As can be seen from the above discussion, in general it would be desirable to be able to scan a probe relative to a surface at a known height to measure a parameter other than topography. In addition, scanning probe systems have the capability to modify or construct surface features on a very fine scale. Typically, such functions may eliminate or are not compatible with a position feedback signal. In the prior art, the scanning probe measurements other than topography, have been carried out or tasks performed while simultaneously sensing the height of the probe. Two prior art patents, which are directed to improvements in scanning which may be used in the present invention, are the Elings and Gurley U.S. Pat. No. 4,889,988 and the Elings and Maivald U.S. Pat. No. 4,954,704.
The Elings and Gurley patent is directed to the use of digitally controlled motion of the probe or sample in scanning probe microscopes and teaches the use of digitally stored position data to better control the scanning motion of the scanning probe microscope. This patent is directed to the measurement of topography and not to the measurement of sample surface properties other than topography. However the present invention may use the digitally controlled motion of the Elings and Gurley patent for the scanning probe of the present invention and the teachings of this patent is therefore incorporated into the present application.
The Elings and Maivald patent teaches a method of rapid scanning in which stored digital topographical data is used to control the return motion of a probe so that it can move rapidly above the sample surface without any risk or damage but still allow the probe to be quickly positioned for the next scan. However, this patent does not have any indication of the measurement of properties of the sample other than topography during the return scan or any subsequent scan. This patent may also be used in the present inventions to control the scanning and its teaching are incorporated into the present application.
Both the Elings and Gurley and Elings and Maivald patents, although directed to improvements in scanning and therefore useful in the present invention, do not anticipate the measurement of properties of the sample surface other than topography or performance of a task at the sample surface, which is the focus of the present invention.